First measurement of the bs0 semileptonic branching ratio to an orbitally excited d**s state, Br(Bs0 → Ds1-(2536)μ+νX)

In a data sample of approximately 1.3 fb-1 collected with the D0 detector between 2002 and 2006, the orbitally excited charm state Ds1±(2536)has been observed with a measured mass of 2535.7 ± 0.6(stat) ± 0.5(syst) MeV/c2 via the decay mode Bs0 → Ds1-(2536)μ+νX followed by Ds1±(2536) → D*±KS0. By normalizing to the known branching ratio Br((bar b) → D*- μ+νX) and to the number of reconstructed D* mesons with an associated identified muon, a first-ever measurement is made of the product branching ratio ((bar b) → Ds1-(2536)μ+νX) · Br(Ds1- → D*-KS0). Assuming that Ds1-(2536) production in semileptonic decay is entirely from Bs0, an extraction of the semileptonic branching ratio Br(Bs0 → Ds1-(2536)μ+νX) is made. Comparisons are made with theoretical expectations